Traditional x-ray imaging techniques employ integrating detectors that detect the total intensity of a transmitted beam through an object to be imaged. Because different materials (such as bone and tissue) attenuate the x-ray beam by different amounts, the resulting detected intensity will vary accordingly. In order to improve the ability to discriminate between materials, spectral x-ray imaging techniques have been developed. These techniques allow for enhanced discrimination between different materials by taking advantage of the energy dependence of the attenuation of x-rays. Because the attenuation of x-rays by a material has an energy (spectrum) dependence that is different among materials, spectral information can be used to enhance discrimination between materials. For example, two materials that have the same attenuation at one x-ray energy may have distinct attenuations at another x-ray energy, thereby allowing them to be discriminated.
One older approach to spectral imaging involves imaging an object using an x-ray beam switched between two distinct energies and detecting images using traditional integrating detectors. Combining the images at these two energies allows for improved discrimination between characteristics of materials in the object being imaged. One disadvantage of this approach is that the two images are not simultaneous.
Another approach to spectral imaging uses a single broad spectrum kVp x-ray beam and energy-discriminating detectors. These are described in U.S. Pat. No. 4,029,963 to Alvarez and Macovski, incorporated herein by reference. The patent describes one approach wherein the detector is divided into layers in the direction of x-ray travel. Since lower energy x-rays tend to be absorbed more easily than higher energy x-rays, the front layer will detect x-rays with a lower effective energy than the back layer. This has been generalized to more than two layers, e.g. see Stevens and Pelc, Medical Physics, vol 27, pp 1174-84, 2000. While these layered detectors provide spectral information, the energy separation is not ideal and there is significant overlap in the spectra detected in the various layers.
Photon counting x-ray detectors (PCXD) with energy discriminating capabilities are the most promising type of detector for this approach to spectral x-ray imaging, primarily because they provide high dose-efficiency due to elimination of electronic noise, and the potential for energy discrimination, the latter being especially important for spectral imaging.
Photon counting x-ray detectors, however, have a relatively slow counting rate that causes count losses and pulse pile-up. To reduce photon saturation effects caused by the insufficient counting speed, a multi-layer (“in-depth”) detector system with separate read-out channels has been proposed to improve the speed. Specifically, Roessl et al. proposed an edge-on cadmium-zinc-telluride (CZT) detector with multiple layers of different thicknesses and individual read-out channels for each layer. Improving upon this approach, Bornefalk et al. validated the feasibility of edge-on silicon (Si) strip detectors, which are more economical and easier to fabricate, to achieve even higher pile-up-free count rate. In effect, these layered detectors are designed to solve the saturation problem of photon-counting type detectors by distributing the detection load among separate material layers. The separate signals from the different read-out channels are combined, and the resulting net signal, which is similar to that from a single fast-counting thick layer, is used for material characterization.
Despite progress in overcoming the limited counting rate of PCXDs, there remain other limiting factors of energy discriminating PCXDs. For example, one such limitation is the imperfect energy response of the detector material. Detected photons may produce signals lower than their actual energy due to Compton scatter events, K-escape, charge-sharing, and other phenomena. For example, in silicon (Si) a large fraction of the detected signals might be Compton scatter events while K-escape can be important in cadmium-telluride (CdTe) and cadmium-zinc-telluride (CZT) detectors. Some of these degradations are seen in detectors operating in other modes (e.g., energy integrating), but become more critical in PCXDs due to the expected energy discrimination.
In view of the above, there remains a need for further improvements in spectral x-ray imaging detectors and imaging techniques.